The NIEHS knock out core (KOC) is a service orientated facility, which aims at helping investigators at NIEHS develop animal models to further their research and fulfilling the mission statement of NIEHS. The KOC aids investigators in generating transgenic mouse models specific for their research needs; including traditional knockouts, conditional knockouts, and knock-ins. It also provides assistance in establishing mutant cell lines for in vitro studies, dissecting early stage embryos, and establishing genotyping strategies for both novel mouse lines and existing lines. Each project in the core is a long-term commitment, and numerous projects are at different stages of completion at any given time of the year. The core typically works closely with investigators from project inception, which starts with meeting with the PI's group to determine not only technical aspects of the proposed project but also to provide scientific guidance in generating the most appropriate mouse model. The KOC recently hired a molecular geneticist to aid investigators that lack expertise in generating targeting vectors and developing screening strategies. With the increased personnel, the KOC has been able to generate transgenic mouse lines using the ROSA26 locus with substantial success. This has been done through conventional homologous recombination and recombinase mediated cassette exchange (RMCE), which allows for insertion of single copy transgenes into the ROSA26 locus. Although the KOC is focused on the scientific support of investigators at the institute, this is done with financial consideration, which includes exploiting the ROSA26 locus to generate classic transgene mouse lines in-house, being available to meet with investigators to optimize mouse breeding strategies, and overseeing complex mouse colonies such as the COX-2 colony, where closely working with CMB and guiding them to develop appropriate screening and breeding strategy to maintain the desired mouse colony saved substantial amount of resources for the DIR. In addition to meeting investigators need to generate mutant mouse models, KOC works closely with CMB/ACUC and provides the necessary expertise to help develop efficient breeding strategies to optimize animal use and minimize cost. KOC has recently expanded its in-house embryo storage system to CMB for long term storage of orphaned mouse lines and short term storage of recently shipped embryos for cryo-recovery. This has reduced the total DIR cost by discontinuing yearly storage payment for low priority and orphan embryo lines from Taconic, Charles River, and Jackson Lab. Additionally, the KOC recently assumed the role of cryo-recovery and embryo transfer from CMB QAL. This will result in cost savings to NIEHS and faster importation of mouse lines into NIEHS from non-commercial source. Finally, NIEHS KOC played a significant role in providing research support to the DIR investigators through close collaborative efforts that resulted in helping them publishing their research articles in many of those quality peer reviewed journals.